Several fixes to improve performance, stability and security, see the Security Advisory.For a more detailed list of bug fixes, see the Rumbling Edge for a Thunderbird-focused list, or the complete list of changes in this version.

You may also be interested in the Thunderbird 3.1.7 release notes for a list of changes in the previous version. See the Thunderbird 3.1 release notes for a list of what is new in Thunderbird 3.1.

Interestingly, I was alerted of the Thunderbird's new version, minutes after I opened it, but not Firefox, which I had open for nearly 30 min. As for this Firefox, I am using the portable version, and no update was available, yet, for either Firefox or Thunderbird.